154832

So you have this grid:

N - -
- - 7
- - 14

You can't put the largest numbers together because that would mean that the grids rows columns and diagonals would all equal X. With the smaller numbers you cannot make X.
Also the middle number should be around the middle as it has a bigger probability of working.
So say you put 11 in the middle.

N - -
- 11 7
- - 14
So then because on the diagonal I had two large numbers 11 & 14 I put the smallest number (8) at the end.
8 - -
- 11 7
- - 14

You know know that each row sums to 33.

8 13 12
15 11 7
10 9 14

So therefore N = 8